Electrophotographic charge generation layer

ABSTRACT

An electrophotographic plate comprising a conductive substrate, a charge transport layer and a charge generation layer consisting essentially of from 5 to 35 percent by weight tellurium and from 0.5 to 20 percent by weight arsenic, with the substantial balance being vitreous selenium.

United States Patent Chiou 1 Jan. 21, 1975 4] ELECTROPHOTOGRAPHIC CHARGE3,355,289 11/1967 Hall et a1, 96/1.5 GENERATON LAYER 3,484,237 12/1969Shattuck.... 96/1.5 3,712,810 1/1973 Ciuffini 96/1.5 [75] Inventor:Charles Chiou, San Jose, Calif. [73] Assignee: International BusinessMachines Own-FER PUBLICATIONS Corporation, Armonk NY. IBM TechmcalD1sc1osure Bulletin Vol, 19 N0. 9 p.

2781. [22] Filed: Mar. 31, 1972 [21] Appl. No.: 239,984 PrimaryExaminerNorman G. Torchin Assistant Examiner-John L. Goodrow 52 U.S. c1.96/1.S, 252/501 Walsh [51] Int. Cl 603g 5/02 [58] Field of Search96/1.5; 252/501; 313/94; [57] ABSTRACT 29/1 H An electrophotographicplate comprising a conductive substrate, a charge transport layer and acharge gener- [56] References cued ation layer consisting essentially offrom 5 to 35 per- UNITED STATES PATENTS cent by weight tellurium andfrom 0.5 to 20 percent 2,863,768 12/1958 Schaffert 96/15 by weightarsenic, with the substantial balance being 3,041,166 6/1962 Bardeen96/l.5 vitreous elenium. 3,077,386 2/1963 Blakney et a1. 96/1.53,350,595 10/1967 Kramer 313/94. 6 Claims, N0 DrawingsELECTROPHOTOGRAPIIIC CHARGE GENERATION LAYER FIELD OF THE INVENTIONPRIOR ART The use of vitreous selenium in electrophotography is wellknown, as shown for example in U.S. Pat. No. 2,970,906. The addition oftellurium to selenium is shown in U.S. Pat. No. 2,745,327. The additionof arse- I nic to selenium is shown in U.S. Pat. No. 2,803,542. Thethree component material which is the subject of the presentapplication, however, it not disclosed in the prior art, and nothing inthe prior art suggests the advantages obtained using a charge generationlayer having the specified amounts of the three ingredients.

SUMMARY OF THE INVENTION In electrophotography, the photoconductiveinsulator performs two functions, namely charge generation and chargetransport. In themost commonly used processes, both functions areperformed by a single layer, for example a layer of vitreous selenium.These two functions may each be performed by a separate layer, and it iswith such systems that the present invention deals.

The present invention provides novel charge generation layers for use inelectrophotographic plates. These novel charge generation layers consistessentailly of from 5 to 35 percent by weight tellurium, and from 0.5 to20 percent by weight arsenic, with the substantial balance beingvitreous selenium. The most preferred composition is from 25 to 30percent tellurium and from 2 to percent arsenic. This three-componentcomposition possesses unexpected advantages as a charge generationlayer. In particular, it is extremely versatile. Depending upon thecharging conditions and upon the nature of the charge transport layer,the charge generation layers of the present invention can generateeither electrons or holes. Furthermore, they are in some cases suitablefor use on top of charge transport layers, and in other cases, beneathcharge transport layers. In addition, the novel charge generation layersof the present invention may be used with either positive charging ornegative charging.

The versatility of the charge generation layers of the present inventionis such that they can, for example, be used to generate holes when theyare positively charged as an overcoat for a vitreous selenium chargetransport layer. In like manner, the charge generation layers of thepresent invention may be used to generate electrons when they arenegatively charged and rest upon a layer of organic photoconductor suchas described in U.S. Pat. No. 3,484,237 of Shattuck and Vahtra.

When the charge generation layers of the present invention areovercoated with a charge transport layer of polyvinylcarbazole or alayer of polyester containing oxadiazole, they are useful when chargedin the negative mode. In like manner. the novel charge generating layersof the present invention may be overcoated by a layer of polyestercontaining trinitrofluorenone and charged in the positive mode.

In some instances it may be of advantage to employ a vitreous seleniumtransport layer having as an interface with the charge generation layera gradient composition comprising selenium and amounts of arsenic andtellurium which decrease with increasing distance from the chargegeneration layer surface. It is thus seen that, depending upon theproper choice of charge transport layer, the charge genera-tion layersof the present invention may be either overcoated onto the chargetransport layer. or overcoated by the charge transport layer, and may becharged in either the negative mode or the positive mode. This extremeversatility was totally unexpected, and constitutes one of the majoradvantages of the present invention.

The most outstanding advantage of the present invention is its very highsensitivity, over a very broad spectral range, when it is used as acharge generation layer with a suitable charge transport layer. Anadditional advantage of the present invention is that it is suitable foruse on a wide variety of substrates, both rigid and flexible, and ofmany different shapes.

It is essential that the specified percentages of arsenic and telluriumbe present. When less than 5 percent by weight of tellurium is containedin the charge generation layer, it loses sensitivity. On the other hand,when the tellurium concentration is above 35 percent, unacceptable darkdecay occurs. Unless at least 0.5 percent by weight arsenic is present,surface crystallization of the layer may occur. On the other hand, whenthe arsenic concentration is greater than 20 percent by weight, theresidual charge is too high.

In general it is preferred that the charge generating layers of thepresent invention be from about 0.02 to about 1.5 microns thick, mostpreferably from about 0.05 to 1.0 microns. In most cases, the chargetransport layers will be much thicker, for example from about 8 to about15 microns. When the charge transport layer is on top of the chargegeneration layer, the charge transport layer must be at least partiallytransparent.

When the charge generation layers of the present invention are used inelectrophotographic plates, in most cases it is preferred that asuitable barrier layer also be employed. Many types of barrier layersare well known in the art. They serve the functions of holding thecharge and preventing carrier injection from the conductor substrate.Barrier layers may be organic, for example, polyamide or polyurethane,or they may be inorganic, for example, aluminum oxide. In many cases thealuminum oxide film normally present on an aluminum conductive substrateis a suitable barrier layer.

The following Examples are given solely for purposes of illustration andare not to be considered limitations on the invention, many variationsof which are possible without departing from the spirit or scopethereof.

EXAMPLE I A positive charging electrophotographic plate, which consistsof an I [L Se (68 wt. %)-Te (30 wt. %)-As (2 wt. generation layerovercoated onto a 35 to 50p vitreous Se transport layer on an anodizedAl substrate, is prepared by using vacuum evaporation. The substrate isheld at a temperature near 70C when the evaporations are conducted in ahigh vacuum system at a pressure of approximately I X 10" torr. The Sefilm is vacuum deposited by heating the Se source to approximately 290Cfor approximately 30 minutes. This is immediately followed by thedeposition of the SeTeAs overlayer using flash evaporation of prealloyedgranules. The crucible temperature for flash evaporation of SeTeAs isnormally maintained at about 500C. A Sloan AMNl-II is used to monitorthe deposit rates and film thicknesses of the Se and SeTeAs films. Atthe completion of the two consecutive depositions, theelectrophotographic plate is cooled in situ to 25C.

To assess image quality, this plate is corona charged to a positivepotential of about 900 volts, and then exposed to a photocopy greenlight source at about 0.28 micro joules/Cm to form a latentelectrostatic image on the plate surface. The latent image is thendeveloped and transferred to a sheet of paper. A good quality imagereproduced from the original is thus obtained.

Quantum efficiency measurements indicate that this electrophotographicplate exhibits a broad spectral response with peak quantum efficienciesup to l, 0.8, and 0.5 at 4,000, 5,000 and 6,000 A respectively and showsweak field dependence of the quantum efficiency.

To check for surface crystallization, at 1p. Se (68 wt. percent) Te (30wt. percent) As (2 wt. percent) film after an anneal treatment at 56Cfor 88 hours is examined by X-ray diffraction. The results of detectingno crystallinity in the annealed film suggest the increased resistanceto surface crystallization of the film due to the presence of As.

EXAMPLE II A negative charging electrophotographic plate which consistsof a 0.1;. Se (68 wt. %)-Te (30 wt. %)-As (2 wt. generation layerovercoated onto a 1 transport layer of the organic photoconductordisclosed in U.S. Pat. No. 3,484,237, on an aluminized Mylar substrateis prepared. (Mylar is duPonts brand of polyethyleneterephthalate.) thevacuum deposition of the SeTeAs film is carried out in the same manneras in Example 1 above except that in place of the 70C substratetemperature a substrate temperature of 45C is used. This plate is coronacharged to a negative potential of about 700 volts and exposed to aphotocopy green light source at about 0.87 micro-joules/cm to form alatent electrostatic image on the plate surface. The electrostatic imageis then developed and transferred to a paper. The image thus obtainedshows excellent quality with minimal background.

EXAMPLE III A positive charging electrophotographic plate, whichconsists of an aliminized Mylar substrate, a 0.3;. du Pont Elvamide 8061(polyamide) barrier layer, an 1 u Se(68 wt. percent)-Te (30 wt.percent)-As (2 wt. percent) charge generation layer and a 10p. 1 to 1 byweight trinitrofluorenonepolyester resin transport layer, is prepared.The SeTeAs charge generation layer is fabricated as in Example 1 above.The transport layer is prepared by dissolving the 2, 4,7-trinitrofluorenone (TNF) in tetrahydrofurane (THF) solvent with theGoodyear Vitel PE-ZOO polyester resin. Application of the transportlayer, overlaying on the SeTeAs generation layer, is normallyaccomplished by using a meniscus coating technique.

This electrophotographic plate is corona charged to a positive potentialof about 750 volts and exposed to a photocopy green light source atabout 0.41 micro joules/cm 2 to form a latent image on the platesurface. This latent image when developed and transferred to a paper,shows excellent quality with little background.

EXAMPLE IV A positive charging electrophotographic plate is prepared asin Example lll above except that in place of the 1p. Se(68 wt %)-Te (30wt. %)-As (2 wt. film, an In Se(70 wt. %)-Te (20 wt. %)-As (10 wt. isused as the generation layer. This plate is corona charged to a positivepotential of about 700 volts. Upon exposure to a tungstenhalogen liggtsource at about 0.89 micro joules/cm the plate is observed to dischargeto a potential of about 200 volts.

EXAMPLE V A negative charging electrophotographic plate, which consistsof an anodized Al substrate, an lp. Se (73 wt. %)-Te (25 wt. %)-As (2wt. charge generation layer and a lip. PE 200 polyester (49.75 wt. )-2,-5-bis-dimethylamine-p-phenylene-l ,3 ,4 oxadiazole (49.75 wt. %)-TNF(0.5 wt. is prepared as in Example 111 above. This plate is negativelycorona charged and then exposed, developed and the toned imagetransferred in the same manner as the plate in Example 111. The imagethus obtained shows good quality with a slight background. Quantumefficiency measurements indicate that this plate shows a broad spectralresponse with peak quantum efficiencies up to 0.56, 0.54 and 0.29 at4,500, 5,000 and 6,000 A respectively.

What is claimed is:

1. An electrophotographic plate comprising a conductive substrate, acharge transport layer, and a charge generation layer consistingessentially of from 5 to 35 percent by weight tellurium and from 0.5 to20 percent by weight arsenic, with the substantial balance beingvitreous selenium.

2. An electrophotographic plate as claimed in claim 1 wherein the chargegeneration layer comprises from 2 to 10 percent by weight arsenic andfrom 25 to 30 percent by weight tellurium.

3. An electrophotographic plate as claimed in claim 1 wherein the chargegeneration layer covers a charge transport layer of vitreous selenium.

4. An electrophotographic plate as claimed in claim 1 wherein the chargegeneration layer covers a charge transport layer comprising an organicphotoconductor.

5. An electrophotographic plate as claimed in claim 1 wherein the chargegeneration layer is covered by a charge transport layer comprisingpolyester resin and trinitrofluorenone.

6. An electrophotographic plate as claimed in claim 1 wherein the chargegeneration layer is covered by a charge transport layer comprisingoxadiazole and a polyester resin.

2. An electrophotographic plate as claimed in claim 1 wherein the chargegeneration layer comprises from 2 to 10 percent by weight arsenic andfrom 25 to 30 percent by weight tellurium.
 3. An electrophotographicplate as claimed in claim 1 wherein the charge generation layer covers acharge transport layer of vitreous selenium.
 4. An electrophotographicplate as claimed in claim 1 wherein the charge generation layer covers acharge transport layer comprising an organic photoconductor.
 5. Anelectrophotographic plate as claimed in claim 1 wherein the chargegeneration layer is covered by a charge transport layer comprisingpolyester resin and trinitrofluorenone.
 6. An electrophotographic plateas claimed in claim 1 wherein the charge generation layer is covered bya charge transport layer comprising oxadiazole and a polyester resin.